Ethylene oxide production



1950 M. L. COURTER ETAL 2,960,511

ETHYLENE 0x102 PRODUCTION Filed Feb. 16, 1959 7 v 1 M/ B FIG.

LO 2 O MOLE RATIOZ ETHYLENE/OXYGEN P22 ZOEbDaOma mQXO mZm4 Ihm Qmhmm ZOo M 20 ETHYLENE CONVERSION FIG.

INVENTORS:

MARTlN L. COURTER JOHN W. MECQRNEY THEIR AGENT ETHYLENE on PRODUCTION Martin L. Courier, Walnut Creek, and John W. Mecorney, Lafayette, Calif., assignors to Shell Oil Qompany, a corporation of Delaware Filed Feb. 16, 1959, Ser. No. 793,305

Claims. (Cl. 260348.5)

This invention relates to the production of ethylene oxide by the direct oxidation of ethylene with molecular oxygen, and relates more particularly to the production of ethylene oxide by molecular oxidation of ethylene in the presence of catalysts comprising metallic silver.

Ethylene oxide is produced by the controlled, silvercatalyzed incomplete oxidation of ethylene with molecular oxygen. Ethylene oxide will often constitute but a relatively small part of the total effluent stream leaving the oxidation reaction zone. This is generally due to the fact that inert diluent materials are ordinarily introduced into the reaction zone in varying quantities together with reactants. Such diluents include, for example, fixed gases such as nitrogen, argon and the like, as well as carbon dioxide, saturated hydrocarbons, etc., introduced into the reaction zone with make-up charge of reactants or with streams recycled from within the system. Inert diluents such as nitrogen, steam, etc., are at times deliberately added from an outside source to obtain optimum reaction conditions and/or to avoid the presence of mixtures which are dangerously explosive under specific conditions employed. It is readily apparent, therefore, that any substantial reduction in yield of ethylene oxide, or loss of ethylene oxide within the system due to its decomposition, will bring the production rate below the minimum commensurate with practical scale operation.

Inability to maintain the ethylene oxide production rate at a desired high value over prolonged periods of operation has been a serious problem in the practical scale operation of many of the processes disclosed heretofore. Expedients resorted to heretofore to solve this problem are directed, not only to the use of carefully controlled operating conditions, but to the use of reaction zone surfaces consisting of materials of construction which are either exceedingly costly as such, or difficult and/or costly to install, maintain and/ or replace.

The use of reaction zones and adjacent parts of apparatus consisting at least in part of stainless steels or similar costly alloys, linings of noble metals, or alloys thereof, carborundum, and the like, was often considered essential heretofore to the attainment of an ethylene oxide production rate commensurate with practical scale operation. Since the controlled ethylene oxidation reaction is highly exothermic, a high ratio of reactor surface to catalyst volume is required to remove large amounts of heat at a rate enabling the maintaining of desired operating conditions. The inordinately high cost of initial installation, in addition to the greater expense of main tenance and/ or replacement often accompanying the use of such relatively costly materials, obviously is a severe deterrent to efiicient commercial scale production of ethylene oxide by the direct controlled oxidation of the readily available ethylene.

It is therefore an object of the present invention to provide an improved process enabling the more efiicient production of ethylene oxide, wherein the need for reac- Zfihfifiil Patented Nov. 15, 1960 tion systems of relatively costly materials of construction, such as alloy steels, for example, stainless steels, and the like, are dispensed with. Another object of the invention is the provision of an improved process enabling the more eflicient direct oxidation of ethylene to ethylene oxide wherein the oxidation is executed in a reaction zone of low cost carbon steel, or ordinary mild steel. Other objects and advantages of the invention will become apparent from the following detailed description thereof made with reference to the attached drawing forming a part of the present specification.

In accordance with the invention ethylene oxide is obtained with substantially improved efficiency by the silver-catalyzed, controlled, direct oxidation of ethylene with molecular oxygen by executing the oxidation in a reaction zone wherein surfaces in direct contact with the reaction mixture are of ordinary mild, or carbon, steel while maintaining a molar excess of ethylene over free oxygen in the reaction zone throughout the course of the oxidation.

In the production of ethylene oxide by the silver-catalyzed controlled oxidation of ethylene with molecular oxygen, reactants comprising ethylene and molecular oxygen are passed over a catalyst containing metallic silver as the effective catalytic component, at a temperature of from about to about 450 C. Pressures in the range of from about atmospheric to about 500 lbs. per square inch, and at times somewhat higher, are employed.

Diluent materials such as, for example, nitrogen, carbon dioxide, steam, argon, saturated hydrocarbons, etc. are generally present in varying amounts. Such diluents may be introduced into the system willfully from an outside source or they may be recycled from within the system alone or together with unreacted materials.

An agent capable of exerting 'a favorable effect upon the catalytic oxidation reaction is optionally employed. Such agents are disclosed in the prior art literature relating to the controlled oxidation of ethylene and comprise, for example, halogen-containing compounds, such as chlorinated hydrocarbons.

The present finding that the silver-catalyzed controlled oxidation of ethylene to ethylene oxide can be executed with improved efliciency in reactors of low cost carbon steel is based upon the discovery of a pronounced effect of reactor surface upon the relationship between the ethylene oxide production rate and the mole ratio of ethylene to molecular oxygen in the reaction mixture at the silver-catalyzed oxidizing conditions. It has now been found that the rate of ethylene oxide production generally remains constant and often, depending upon specific conditions employed, declines with increase in the mole ratio of ethylene to molecular oxygen in the reaction mixture when the reactor surfaces consist essentially of alloy steels such as stainless steel or similar alloys used heretofore for the purpose. It was further found, however, that quite surprisingly, in the presence of surfaces of ordinary mild, or carbon steels, the ethylene oxide production rate will generally increase quite markedly with increase in mole ratio of ethylene to m0- -lecular oxygen in the reaction mixture. It was also found that when the mole ratio of ethylene to molecular oxygen in the reaction mixture is at a value below about 1 there usually is indeed advantage in the use of the more costly alloys, such as stainless steels. However, when the ratio of ethylene to molecular oxygen is maintained above about 1, preferably above about 1.3, not only does it become possible to use reactor surfaces of low cost carbon, or ordinary mild steel, but it generally becomes definitely advantageous to do so.

Example I In a series of separate operations ethylene oxide was produced by passing ethylene in admixture with molecular oxygen through a carbon steel (ASTM-A-179) tubular reactor containing a silver metal catalyst. The catalyst employed consisted essentially of metallic silver supported upon alumina. The reaction was executed at a temperature of about 259 C. and about 115 p.s.i.g. The runs of the series were executed at substantially identical conditions with the exception that a different ratio of ethylene to molecular oxygen was maintained in the reaction mixture used in each. The results in each of the operations were determined and recorded in terms of pounds of ethylene oxide production per pound of catalyst per hour. Plotting this production rate against mole ratio of ethylene to molecular oxygen in the reaction mixture results in curve A reproduced in the Fig. 1 of the attached drawing.

In a second series of operations ethylene oxide was produced by the silver metal-catalyzed, controlled oxidation of ethylene under substantially the same conditions used in the foregoing series of experiments. In the second series of runs, however, the tubular reactor employed was of stainless steel (schedule 40-304 stainless steel). Plotting ethylene production rate against mole ratio of ethylene to molecular oxygen maintained in the reaction mixture for the second series wherein the stainless steel tubular reactor was used, results in curve B reproduced in Fig. 1 of the attached drawing.

From the foregoing Example I it is apparent that not only can the use of costly alloy steels be dispensed with, but the desired silver-catalyzed, controlled oxidation carried out with substantially improved efliciency, in terms of ethylene oxide production rates, in ordinary mild, or carbon steels, if operating conditions are controlled to maintain a mole ratio of ethylene to molecular oxygen in the reaction mixture above about 1, and preferably above about 1.3. This is further evidenced by the following example showing the efl'ect of carbon steel and alloy steel reactor surfaces upon ethylene oxide yield in comparative silver-catalyzed ethylene oxidations.

Example II In a plurality of operations C ethylene oxide was produced by passing ethylene in admixture with molecular oxygen over a silver metal catalyst in a tubular reactor of ordinary mild steel (ASTMA-17B Type A steel- 12 gauge). The oxidation was executed at a temperature of about 250 C. and about 115 p.s.i.g. The catalyst employed consisted essentially of metallic silver supported upon an alumina support. About 5 parts per million of ethylene dichloride was added to the charge. The mole ratio of ethylene to molecular oxygen in the charge was maintained at 1.7 throughout the course of the operation. The charge to the process was diluted with nitrogen to maintain the ethylene concentration in the total charge to the reactor at about 15%. The production of ethylene oxide by the silver-catalyzed, controlled oxidation of ethylene with molecular oxygen was repeated in a separate plurality of operations D under substantially the same conditions employed in the operations of operation C but with the exception that a tubular reactor consisting of alloy steel (1" schedule 40304 stainless steel) was employed in the runs of series D.

Ethylene convertedand ethylene oxide yield were determined for each operation. In the Fig. 2 of the attached drawing the curves C and D are the curves obtained by plotting ethylene conversion against ethylene oxide yield for the operations carried out in the tubular reactor of plain carbon steel (C) and stainless steel (D), respectively.

Essential to the attainment of the objects of the invention is the maintaining of a molecular excess of ethylene over molecular oxygen at all times throughout the course of the oxidation. The mole ratio of ethylene to oxygen is preferably maintained at a minimum value,

of about 1.3, and still more preferably at a minimum of about 1.5. In general a mole ratio of ethylene to oxygen of about 3 need not be exceeded. Higher ratios, for example, up to about 7 and higher, may be employed, however, within the scope of the invention.

Applicants discovery with respect to the profound effect exerted upon the efliciency of the process by the relationship existing between the nature of the reactor surface in contact with the reaction mixture and ratio of ethylene to molecular oxygen present under the ethylene oxidizing conditions employed, is substantiated by the further discovery that when the ratio of ethylene to oxygen is above about 1.3 there is no substantial adverse efiect of ordinary mild, or carbon steel upon the stability of ethylene oxide at the temperature prevailing during the oxidation. On the other hand, substantial decomposition of ethylene oxide takes place in the presence of carbon steel, or mild steel, surfaces at these temperatures when the ratio of ethylene to oxygen is below the above-defined critical value. This is evidenced by the following example.

Example 111 In an operation B an ethylene oxide-containing reaction mixture, obtained by the silver metal-catalyzed controlled oxidation of ethylene with molecular oxygen using a mole ratio of ethylene to oxygen of 2, at 235 to 255 C. and about p.s.i.g., contains 1.48 mole percent ethylene oxide. The rest of the reaction mixture consisted essentially of ethylene, oxygen, Water, nitrogen, argon, carbon dioxide and ethane. The mole ratio of ethylene to molecular oxygen in the reaction mixture was 3. This reaction mixture was passed through an empty tubular reactor consisting of a bundle of carbon steel tubes (1015 SAE) arranged in parallel. The tubular reactor presented a surface to volume ratio of 22.2 sq. in. per cu. in. The tubes were maintained at 235 to 255 C. by external heating. The reaction mixture was passed through the tubes with a residence time of second at a pressure of 185 p.s.i.g. The ethylene oxide content of the efiiuent stream was determined after cooling. It was found that more than 98% of the ethylene oxide charged to the tubular reactor was recovered unchanged. The operation was then repeated in six additional experiments. The average ethylene oxide recovery for the six experiments was 98.6%.

In a separate run F, a reaction mixture obtained by controlled silver-catalyzed ethylene oxidation using a mole ratio of ethylene to molecular oxygen of 0.3 at 235 to 255 C. and about 185 p.s.i.g., contains 0.7 mole percent ethylene oxide. The rest of the reaction mixture consisted essentially of ethylene, oxygen, water, nitrogen, carbon dioxide, argon and ethane. The mole ratio of ethylene to molecular oxygen in this reaction mixture was 0.23. This reaction mixture was passed through the tubular reactor employed in the foregoing operation E at substantially the same conditions used in that operation. Determination of the ethylene oxide content of the cooled efiluence showed that only 86.5% of the ethylene oxide charged to the tubular reactor was recovered.

It is seen from the foregoing Example III that as much as 12% of the desired ethylene oxide is lost in a single passage through the tubular reactor when departing from the permissible ratio of ethylene to molecular oxygen defined herein.

The invention may be applied to the silver metalcatalyzed controlled oxidation of ethylene to ethylene oxide using reaction zones comprising a single reactor, or a plurality of reactors, of restricted or enlarged crosssectional areas, in series or parallel flow. Essential to the attainment of the objects of the invention is that the surface of the reaction zone in direct contact with the reaction mixture be of ordinary mild, or carbon steel. By the term carbon steel, as used in the present specification and attached claims is meant the broad class of steels known as carbon steels in the industry as defined in Kirk-Othmer The Encyclopedia of Chemical Technology, vol. 12, page 816, 1954 edit. These are intended to include the carbon steels often referred to as plain carbon steels, and mild steels in the industry. The carbon steels generally comprise in addition to iron and carbon one or more of the following: Si, Mn, P, S, and Cu. Other components normally encountered in minor amounts in commercial grades of carbon steels may be present. They form a class which is distinct from the steels known as alloy steels in the industry and which comprise such elements as Ni, Cr, Mo, Ti, Vn, etc., as essential components thereof. The alloy steels, unsuited to the attainment of the objects of the invention com prise, for example, stainless steel, high-chromium steel, nickel-steels, further represented by ASTM type 410, ASTM 430T, V2A steels, and the like.

Carbon steels suitably employed within the scope of the invention comprise those identified by the standard identifying S.A.E. numbers having a 1 as the first (left hand) digit in the S.A.E. numbering system for steels. (See S.A.E. Handbook, 1955, Soc. of Automotive Engineers, Inc.) Suitable members of the class of carbon steels may be chosen, with regard to specific structural and manufacturing requirements, from the S.A.E. 1006 to 1095 and 1330 and 1335 steels. Specific examples of suitable steels for the purpose of the invention comprise: ASTM-A-178 Type A, ASTM-A479, ASTM-A-285, ASTM-A-106, ASTM-A-53, ASTM-A 83, ASTM-A- 226, SAE-lOlS, SAE-l020, SAE-l025, SAE-l030, SAE 1050, and the like. The invention is, however, not limited in its application to only specific carbon steels and any of the carbon steels may suitably be employed within the scope of the invention.

In a preferred method of carrying out the process of the invention the reaction mixt re is maintained in contact with surfaces of ordinary mild, or carbon steel, and the mole ratio of ethylene to molecular oxygen therein maintained above the value of l, as long as the reaction mixture is maintained at an elevated temperature.

Maintaining the mole ratio of ethylene to molecular oxygen above the above defined critical minimum throughout the course of the oxidation and preferably also through the cooling stage, may be obtained by judicious control of operating conditions, such as, for example, control of the rate of feed of individual reactants into the reaction zone. In a suitable method of operation the mole ratio of ethylene to oxygen in the charge to the reaction zone is maintained in the range of from about 1.3 to about 7, and preferably from about 1.5 to-about 5.5, while maintaining conditions of operation such that the mole ratio of ethylene to oxygen remains in excess of unity at all times throughout the reaction zone. The addition of suitable diluents, for example, nitrogen, carbon dioxide, steam, argon, and the like, to the reactants to maintain desired operating conditions may be resorted to. A part, or all, of the diluent materials so introduced into the reactor may emanate from an outside source, or they may be recycled thereto from within the system. Any part, or all, of the reactants and/ or diluent materials may be introduced into the reaction zone at more than one point thereof. In a preferred method of operation in accordance with the invention the charge to the system is maintained dilute with respect to ethylene concentration. Thus, the ethylene charge to the system may comprise, for example, from about to about 20 mole percent of the total feed to the reaction zone. Higher or lower concentrations may, however, be used within the scope of the invention. Maintaining the desired ethylene concentration is accomplished by the controlled addition of inert diluents, such as, for example, nitrogen, and/or the recycling of inert materials comprising, for example, nitrogen, carbon dioxide, argon, etc. to the reaction zone.

The process of the invention is applicable to batch, semi-continuous or continuous operations. The catalyst employed may be in the form of a solid fixed bed, dense phase, agitated, or fluidized systems.

Example IV Ethylene oxide was produced by passing a reaction mixture containing 14.6 mole percent ethylene, 8.8 mole percent oxygen, the rest of the charge consisting essentially of nitrogen, carbon dioxide and ethane, through a tubular reactor at a reaction temperature of 262 C. and a pressure of about 180 p.s.i. g. at a gas hourly space velocity of 2360. The reactor consisted essentially of an oiljacketed 1.54 in. ID. x 30 ft. tube of carbon steel (ASTM- A-178 Type A). The reactor contained a silver metal catalyst consisting essentially of metallic silver upon an alumina support. The reactor efiiuence was cooled and analyzed. An ethylene oxide yield of 68% was obtained with an ethylene oxide production rate of 122 pounds of ethylene oxide per cu. ft. of catalyst per day.

Example V In a continuous operation, ethylene oxide was produced by passing a reaction mixture containing 14.7 mole percent ethylene, 2.7 mole percent oxygen, the rest of the change consisting essentially of nitrogen, carbon dioxide and ethane, through a tubular reactor at a reaction temperature of 260 C. and a pressure of about 200 p.s.i.g. at a gas hourly space velocity of 1300. The mole ratio of ethylene to oxygen at the inlet to the reactor was therefore about 5.5. The reactor consisted essentially of oil jacketed 1.54 in. ID. x 30 ft. tubes of carbon steel. The reactor contained a silver metal catalyst consisting essentially of metallic silver upon an alumina support. The mole atio of ethylene to oxygen did not diminish in value during the course of the oxidation. The operation was carried out continuously for 16 hours. The reactor eflluence was cooled and analyzed. An ethylene oxide yield of 70% was obtained.

This application is a continuation-in part of co-pending application Serial No. 657,670, filed May 7, 1957, now abandoned.

The invention claimed is:

1. In the production of ethylene oxide by the metallic silver-catalyzed, controlled oxidation of ethylene with molecular oxygen in a reaction zone at a temperature of from about to about 450 C., the steps which comprise effecting said oxidation reaction in a reaction zone wherein substantially all surfaces in direct contact with the reaction mixture consist essentially of carbon steel, and said ethylene and oxygen are introduced into said reaction zone as a gaseous mixture containing from about 10 to about 20 mole percent ethylene and sufiicient molecular oxygen to maintain the ratio of ethylene to oxygen at a minimum value of 1 but not substantially above about 3, the rest of the said gaseous mixture consisting essentially of nitrogen, carbon dioxide, argon and ethane recycled in part from within the system.

2. In the production of ethylene oxide by the metallic silver-catalyzed, controlled oxidation of ethylene with molecular oxygen in a reaction zone at a temperature of from about 150 to about 450 C., the steps which comprise elfecting said oxidation reaction in a reaction zone wherein substantially all surfaces in direct contact with the reaction mixture consist essentially of carbon steel and said ethylene and oxygen are introduced into said reaction zone as a gaseous mixture containing from about 10 to about 20 mole percent ethylene and suflicient molecular oxygen to maintain the ratio of ethylene to oxygen at a minimum value of about 1.3 and below a maximum value of about 3, the rest of said gaseous mixture comprising nitrogen, carbon dioxide and ethane.

3. In the production of ethylene oxide by the silvercatalyzed controlled oxidation of ethylene with molecular oxygen in a reaction zone at a temperature of from about 150 to about 450 C., the steps which comprise effecting said oxidation reaction in a reaction zone wherein substantially all surfaces in direct contact with the reaction mixture consist essentially of carbon steel, introducing ethylene and oxygen into'fsaid reaction zone in a mole ratio of ethylene to oxygen above 1, but not substantially above about 7, and maintaining the mole ratio of ethylene to oxygen above 1 throughout said oxidation.

4. The process in accordance with claim 3 wherein said ratio of ethylene to oxygen in said mixture charged to the reaction zone is maintained in the range of from about 1.5 to about,5.5.

5. In the production of ethylene oxide by the silvercatalyzed, controlled oxidation of ethylene with molecular oxygen in a reaction zone at a temperature of from about 150 to about 450 C., the steps which comprise etfecting said oxidation reaction in a reaction zone wherein substantially all surfaces in direct contact with the reaction mixture are essentially of carbon steel, and maintaining the mole ratio of ethylene to molecular oxygen in the reaction mixture above 1v but not substantially in excess of about 7 throughout thecourse of said oxidation.

References Cited in the file of this patent UNITED STATES PATENTS 2,040,782 van Peski -4 May 12, 1936 2,430,443 Becker Nov. 11, 1947 2,600,444 Sullivan June 17, 1952 2,628,965 Sullivan Feb. 17, 1953 FOREIGN PATENTS v 538,662 Canada Mar. 26, 1957 OTHER REFERENCES Pokrovskii Uspekhi Khim, Vol.21, No. 7, pp. 785-807 1952). g 5 

1. IN THE PRODUCTION OF ETHYLENE OXIDE BY THE METALLIC SILVER-CATALYZED, CONTROLLED OXIDATION OF ETHYLENE WITH MOLECULAR OXYGEN IN A REACTION ZONE AT A TEMPERATURE OF FORM ABOUT 150* TO ABOUT 450*C., THE STEPS WHICH COMPRISE EFFECTING SAID OXIDATION REACTION IN A REACTION ZONE WHEREIN SUBSTANTIALLY ALL SURFACES IN DIRECT CONTACT WITH THE REACTION MIXTURE CONSIST ESSENTIALLY OF CARBON STEEL, AND SAID ETHYLENE AND OXYGEN ARE INTRODUCED INTO SAID REACTION ZONE AS A GESEOUS MIXTURE CONTAINING FROM ABOUT 10 TO ABOUT 20 MOLE PERCENT ETHYLENE AND SUFFICIENT MOLECULAR OXYGEN TO MAINTAIN THE RATIO OF ETHYLENE TO OXYGEN AT A MINIMUM VALUE OF 1 BUT NOT SUBSTANTIALLY ABOVE ABOUT 3, THE REST OF THE SAID GASEOUS MIXTURE CONSISTING ESSENTIALLY OF NITROGEN, CARBON DIOXIDE, ARGON AND ETHANE RECYCLED IN PART FROM WITHIN THE SYSTEM. 